NORSK GEOLOGISK TIDSSKRIPT 36 THE GEOLOGY OF THE ØRSDALEN DISTRICT. ROGALAND, S. NORWAY BY KNUT HEIER A b s t r a c t. The tungsten and molybdenite deposit in the area has been dealt with earlier by the author; this paper deals with the general geology, petrology and rock metamorphism of the region. After the regional geology has been briefly described, the paper deals with the petrographical classification of the rocks exposed. These are amphibolites and two types of quartzo-feldspathic gneisses. Then follows a short description of the rock structures. It is shown how a heavy falding of orogenic nature has affected all the rocks and it is concluded that they must have reacted to the falding in a plastic way. A chapter where the metasomatic processes are discussed follows next and the conclusions reached are that the ane kind of the gneisses may have originated through metasomatism of anorthositic rocks while strong evidence is that the other has been formed by replacement of amphibolites and related rocks. The metasomatism is believed to be synkinematic. The metamorphism is dealt with at same length and it is shown that all the rocks must once have belonged to granulite facies, which is also the PT conditions during which the metasomatism took place. Later a retrograde meta­ morphism has affected the rocks within certain zones which are believed to represent tectonical zones of weakness. A map showing the metamorphic situation of the area is presented and here a division of the rocks into three groups according to differences in metamorphism is undertaken. These are: high amphibolite facies (including granu lite facies), low amphibolite facies, epidote amphibolite-and greenschist facies. It is shown how a division of the rocks into ane of these three groups can be based upon variations in the minerals of the quartzo-feldspathic gneisses alone. The paper ends with same considerations upon the relative age of the Tele­ mark and Egersund formations of pre-Cambrian rocks. Based on the data known today the author considers a contemporaneous origin as being most probable. 168 KNUT HEIER Locality. The area examined is situated in S. Norway and covers the region from brsdalen (Rogaland) in the north-west to Gyadalen (Vest-Agder) in the south­ east, an area of about 45 km2• (fig. 1). Topography and climate. The lake brsdalsvannet stretches for more than 16 km. in a north-easterly direction, and the width never exceeds l km. The altitude of the lake-surface is 64 m but the bottom of the lake is well below sea level. At the south-west end the lake is dammed by a moraine which is supposed to be connected with the glacial deposit known in Norwegian quaternary geology as the Fig. l. Map showing the situ­ Vestfold-raet. As a continuation of the ation of Orsdalen. lake towards north-east the valley of brsdalen rises gently, with a width never exceeding 700 m, for about lO km to Bjordal where the altitude is 212 m. The valley was iceformed and has a typical U-profile. On both sides the mountains rise, in some places nearly vertically, to altitudes between 600 and 800 m (fig. 2). The mountain plateau is ice-scoured and most of the soil has been carried away, leaving excellent exposures for detailed geological mapping. Trees are only found in the valleys and on cliffs which inter­ sect the area and give it a rugged appearance. The topography is strongly dependent upon the rock structures and a study of a topographical map will give the broad outlines of the strike and dip directions of the rocks. The highest top within the area is Mjåvassknuten with an altitude of 843 m. The climate is typical for the Norwegian west coast with much rain in the summer. In normal winters there are no continuously frozen periods in the valley itself, but the mountains are snow-covered from December to April, and on the cliffs the snow lasts even longer. THE GEOLOGY OF THE ØRSDALEN DISTRICT 169 Fig. 2. View of the mountain precipice from the top of the mountain plateau. Schånings mine is seen a little below the top and some houses belonging to the mine are seen down in the valley. The vertical distance from the valley floor to the mountain plateau is at this place about 500 m. (photo, Dr. H. Bjorlykke). Earlier geological work. Not much is published about the geology of the area. The tung­ sten deposits occuring there are briefly mentioned by BR6GGER (1906, 1922), FOSLIE (1925), OFTEDAL, (1948), and ADAMSON and NEUMANN (1952). In connection with a description of the Knaben molybdenite mines, Bugge briefly mentioned the deposit in HoLTEDAHL (1953, pp. 114-116). BARTH (1945) gives a map of the geology of South Norway (l : 300 000) which also covers Orsdalen. In most of these papers the tungsten deposit is just briefly men­ tioned and nothing is said about the general geology around the deposit. Own work. The work was carried out on behalf of A/S Norsk Bergverk, the present owners of the mine, and the field work was done during the summers 1952-53-54. The work was concentrated about the tung- N l l l l White qneines with omphibolitic bonds Gronodioritlc qneisses with numerous omphibolltic bon(h ORSDALEN o few only occosior,ol Minerolized zone, sketched l km THE GEOLOGY OF THE ØRSDALEN DISTRICT 171 sten deposit and this represents a central part of the mapped area. A description of the deposit itself has been given earlier (HEIER 1955a) and it will not be treated any further in this paper. A discussion of some feldspar perthites found within the area has also been previously published (HEIER 1955b). The aim of this paper is to give a description of the general geology of the area. REGIONAL GEOLOGY The area represents a border zone between the Egersund forma­ tion of anorthositic and monzonitic rocks and the more alkalic gneiss­ granites of the Setesdal-Telemark region, both of believed pre-Cambrian age. The oldest name for these rocks was <<The grey gneiss of Dalane>>. BARTH (1945) used the name Birkremite which is the name of a hyper­ sthene bearing granitic rock belonging to the charnockite family, or rat­ her granodioriticrocks of granulite facies affinities, occuring in connec­ tion with the Egersund formation. On the new geological map of Norway by Holtedahl and Dons the neutral name <<gneisses in gene­ ral» is used. The nearest anorthositic body is found about 10 km to the south­ west of the mapped area and towards the northeast we enter the geologically little-known area of gneisses with granitic affinities covering the huge area of Rogaland, Aust-Agder and Setesdal. These rocks are thought to be associated with the pre-Cambrian Telemark granite, the central part of which is about 120 km NE of Orsdalen. In the Orsdalen area we find a heterogeneous complex of mixed rocks or what would generally be named migmatites: composed of basic rocks with gabbroic or rather amphibolitic affinities and quartzo-feldspathic gneisses. The latter can be divided into two groups one of which is of a common granodioritic composition. All these rocks have reacted to the same pressures which have induced a parallel foliation upon them. Quartz veins occur all through the area and seem always to be parallel with the general foliation. Minor pegmatites occur inside weakly foliated granodiorites with an elongation usually vertical to the general rock foliation. Of uncertain age, but obviously younger than the other rocks, is a diabase dyke which is found in the Orsdalen valley, striking 172 KNUT HEIER N70E and cutting through the general rock foliation (not marked on the map). Similar dykes of a young age are frequent within the Egersund formation. As shown on the attached map the rock foliation varies within large limits. The structures will be discussed in some detail in a later section of this paper, and at this stage attention will just be drawn to the fact that the variations are gradual and the rocks are never seen to be crushed or mylonitized where the strike is turning. The rocks must have reacted to the pressure in a plastic manner. PETROGRAPHY A mphibolites. The amphibolites occur as bands and lenses of minor size within the gneisses all over the mapped area. They seem to be most frequent within the central part, where the ore is also found, and can here be followed continuously in the direction of the strike for several 100 meters with a width usually between 10 and 20 meters. Generally they are, however, small and show all transitions towards complete assimilation in the quartzo-feldsapthic gneisses. In the amphibolites the following minerals are found: Quartz (secondary), plagioclase, rhombic and monoclinic pyroxene, horn­ blende, biotite, chlorite, serpentine, epidote, zoisite, garnet, apatite, sphene, zircon and iron ore (usually magnetite, but also hematite and ilmenite may occur). Table l gives the mineral distribution as determined by <<point counten> on thin-sections of 7 amphibolites from different localities within the area. As seen from the table the amount of plagioclase, or rather plagio­ clase plus zoisite1) (all the plagioclases except no. l have undergone a varying degree of saussuritization), is kept nearly constant all through the series. In no. 5 where the plagioclase is of the compo­ sition of nearly pure albite, a heavy saussuritization has taken place, indicating a much more basic primary composition. The plagioclase 1) In the rocks examined I have not been ab le to demonstratewhetherzoisite or clinozoisite is formed by the saussuritization processes. In this paper the term zoisite will be used for the mineral thus formed.